Cold start rough idle gremlin

Hey guys I've always had issues with a rough idle upon a cold start and lately ive been trying to track it down and fix it. after a couple seconds it will rev up then smooth out. lately it seems to have been getting worse so i have started replacing sensors due to the age and mileage (400k).


I've replaced the TPS, IAC, MAP, and both Cam, and Crank Position Sensor. After all of these sensors I have disconnected the battery and let it discharge and hoped it would relearn and solve the issue but it has not. It's a fresh rebuilt 4.0 now and an entirely new ignition system but still having these issues. The only code i have is for a non functioning speedometer and it has a miss every couple cycles of the engine when idling.


Its gotten to the point I thought this was normal for the 4.0 engines until I've seen videos of xjs starting up and running like a normal vehicle should (who would've guessed)



Where else should I be looking and what else should I be testing? thank you guys in advance you are great help and I greatly appreciate it!

That's how mine acted when the injector wiring started shorting out.

Assuming a 1997, please confirm year and engine.

tealxj97 said:

...... I've always had issues with a rough idle upon a cold start ....

Click to expand...

Have you tested the fuel pressure to look for a faulty check valve ? Have you tested the fuel injectors looking for one or more drippy injectors ?

The 97-01 fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.


POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s).

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail, and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably one or more leaky injectors.

This is very interesting actually. i have new 4 hole injectors installed so i don't think its them but i do have a faulty check valve in the fuel pump. I've always had to prime it a time or two before starting and never thought anything more of it, just an inconvenience. Ill look more into this now knowing it could cause a rough idle like I'm seeing so thank you very much! I'll let you guys know what i find.

Ever clean your throttle body?

Yeah its a bored throttle body maybe about a year old.

While you're looking at the check valve, pay close attention to the rubber fuel lines inside the tank. They are prone to split and crack where the clamps tighten, allowing for lower fuel pressure.

Use SAE R10 rated hose for replacement.

Faulty check valve does this. Even priming it will lead to a rough cold start. Just less cranks. I'd just replace the pump and dont get a cheap one. Keep in mind those long hooks for the tank may need to be cut in order to be removed

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